The present disclosure relates to a high-frequency filter that passes therethrough a high-frequency signal of a desired frequency band and attenuates high-frequency signals of the other frequency bands.
High-frequency modules currently used in wireless communication terminals or the like include high-frequency filters. For example, Japanese Unexamined Patent Application Publication No. 2007-67941 describes a noise filter, which is a high-frequency filter. The noise filter described in Japanese Unexamined Patent Application Publication No. 2007-67941 includes a first inductor, a second inductor, a third inductor, and a capacitor.
The first inductor and the second inductor are connected in series to each other between a first terminal and a second terminal. One end of the third inductor is connected to a conductive line that connects the first inductor and the second inductor to each other. The other end of the third inductor is connected to one end of the capacitor. The other end of the capacitor is connected to another terminal. That is, the high-frequency filter is implemented as a T filter circuit including two series inductors (i.e., the first inductor and the second inductor), a shunt inductor (i.e., the third inductor), and a shunt capacitor (i.e., the capacitor).
In the case where a bandpass filter is constituted by the high-frequency filter having the circuit configuration described in Japanese Unexamined Patent Application Publication No. 2007-67941, the other end of the capacitor is conceivably connected to ground, for example.
To increase inductances of the series inductors in this configuration, the first inductor and the second inductor may be connected to have additive polarity. An additive polarity connection is a connection that produces a positive mutual inductance.
In this case, however, the resultant circuit configuration is such that an inductor having an inductance equal to the mutual inductance with the opposite sign (negative mutual inductance) of the first inductor and the second inductor is connected in series to the shunt inductor. Consequently, an inductance of the shunt LC circuit decreases, and filter characteristics deteriorate.
To increase an inductance of the shunt inductor, the first inductor and the second inductor may be connected to have subtractive polarity. A subtractive polarity connection is a connection that produces a negative mutual inductance. In this case, however, the inductances of the first inductor and the second inductor decrease by the mutual inductance, and filter characteristics deteriorate.